How can I set aspect ratio constraints programmatically in iOS?

I have used auto layout for my view controllers. I have set the V and H positions in constraints, but I want to know how can I increase my button size when it changes to 5s, 6 and 6 Plus. This is the way I added constraints for the login button:

NSArray *btncon_V=[NSLayoutConstraint constraintsWithVisualFormat:@"V:[btnLogin(40)]" options:0 metrics:nil views:viewsDictionary];
[btnLogin addConstraints:btncon_V];

NSArray *btncon_POS_H=[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-100-[btnLogin]-100-|" options:0 metrics:nil views:viewsDictionary];
[self.view addConstraints:btncon_POS_H];


NSArray *btncon_POS_V=[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-70-[Title]-130-[lblFirst]-0-[lblSecond]-20-[textusername]-10-[txtpassword]-10-[btnLogin]" options:0 metrics:nil views:viewsDictionary];

[self.view addConstraints:btncon_POS_V];

But my problem is that while it manages the left and right side gap, it's getting stretched in iPhone 6 and 6 Plus since the height is fixed. How can I increase the size according to the screen size? I think this might be the aspect ratio, but how can I set the aspect ratio constraint in code?

Layout Anchors is the most convenient way to set constraints programmatically.

Say you want to set 5:1 aspect ratio for your button then you should use:

button.heightAnchor.constraint(equalTo: button.widthAnchor, multiplier: 1.0/5.0).isActive = true

Here's the full code:

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let button = UIButton(type: .custom)
        button.setTitle("Login", for: .normal)
        button.backgroundColor = UIColor.darkGray
        
        self.view.addSubview(button)
        
        button.translatesAutoresizingMaskIntoConstraints = false
        
        let margins = view.layoutMarginsGuide
        
        button.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 20.0).isActive = true
        button.trailingAnchor.constraint(equalTo: margins.trailingAnchor, constant: -20.0).isActive = true
        button.bottomAnchor.constraint(equalTo: margins.bottomAnchor, constant: -20.0).isActive = true
        button.heightAnchor.constraint(equalTo: button.widthAnchor, multiplier: 1.0/5.0).isActive = true
    }

}

Here're results achieved with code written above. You can see that button keeps its 5:1 aspect ratio across various devices:

Like this. Try once.

[self.yourview setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.yourview addConstraint:[NSLayoutConstraint
                                  constraintWithItem:self.yourview
                                  attribute:NSLayoutAttributeHeight
                                  relatedBy:NSLayoutRelationEqual
                                  toItem:self.yourview
                                  attribute:NSLayoutAttributeWidth
                                  multiplier:(self.yourview.frame.size.height / self.yourview.frame.size.width)
                                  constant:0]];

or in the place of (self.yourview.frame.size.height / self.yourview.frame.size.width) you can use any float value. Thanks.

Swift 3.0 -

self.yourview!.translatesAutoresizingMaskIntoConstraints = false
self.yourview!.addConstraint(NSLayoutConstraint(item: self.yourview!,
                                          attribute: NSLayoutAttribute.height,
                                          relatedBy: NSLayoutRelation.equal,
                                          toItem: self.yourview!,
                                          attribute: NSLayoutAttribute.width,
                                          multiplier: self.yourview.frame.size.height / self.yourview.frame.size.width,
                                          constant: 0))

Swift 3:

yourView.addConstraint(NSLayoutConstraint(item: yourView,
								          attribute: .height,
								          relatedBy: .equal,
								          toItem: yourView,
								          attribute: .width,
								          multiplier: 9.0 / 16.0,
								          constant: 0))

Helper Extension on UIView

extension UIView {
    
    func aspectRation(_ ratio: CGFloat) -> NSLayoutConstraint {
        
        return NSLayoutConstraint(item: self, attribute: .height, relatedBy: .equal, toItem: self, attribute: .width, multiplier: ratio, constant: 0)
    }
}

And usage is:

view.aspectRation(1.0/1.0).isActive = true

You can set "Height constraint" at Design-time in Interface Builder. Just check '"Remove at build time", and it removes when App will running.

[![enter image description here][1]][1] [1]: http://i.stack.imgur.com/fqhSi.png

After that you can add "Aspect Ratio" constraint, for example, in viewDidLoad method.

In Xamain.iOS for "16:9" it's looks like this:

    this.ImageOfTheDay.AddConstraint(NSLayoutConstraint.Create(
            this.ImageOfTheDay,
            NSLayoutAttribute.Height,
            NSLayoutRelation.Equal,
            this.ImageOfTheDay,
            NSLayoutAttribute.Width,
            9.0f / 16.0f,
            0));

Or, as Mahesh Agrawal said:

    [self.ImageOfTheDay addConstraint:[NSLayoutConstraint
                              constraintWithItem:self.ImageOfTheDay
                              attribute:NSLayoutAttributeHeight
                              relatedBy:NSLayoutRelationEqual
                              toItem:self.ImageOfTheDay
                              attribute:NSLayoutAttributeWidth
                              multiplier:(9.0f / 16.0f)
                              constant:0]];

I have try all answers above but didn't work, and this is my solution with swift 3:

let newConstraint = NSLayoutConstraint(item: yourView, attribute: .width, relatedBy: .equal, toItem: yourView, attribute: .height, multiplier: 560.0/315.0, constant: 0)
yourView.addConstraint(newConstraint)
NSLayoutConstraint.activate([newConstraint])  
NSLayoutConstraint.deactivate(yourView.constraints)
yourView.layoutIfNeeded()

Swift 5 solution, Xcode 12.4. Requires ios 9.0+

Simply anchor your height constraint to a certain ratio:

let aspectRatio: CGFloat = 9 / 16 // Example of ratio you want to apply
yourView.heightAnchor.constraint(equalTo: imageView.widthAnchor,
                                 multiplier: aspectRatio).activate()